Cell division plays a crucial role during all phases of plant development. The continuation of organogenesis and growth responses to a changing environment requires precise spatial, temporal and developmental regulation of cell division activity in meristems. Such control of cell division is also important in organs themselves for example, leaf expansion, and secondary growth. A complex network controls cell proliferation in eukaryotes. Various regulatory pathways communicate environmental constraints, such as nutrient availability, mitogenic signals such as growth factors or hormones, or developmental cues such as the transition from vegetative to reproductive. Ultimately, these regulatory pathways control the timing, frequency (rate), plane and position of cell divisions. The regulation of cell division impacts a variety of developmental pathways including transformation and plant regeneration.
Current transformation technology provides an opportunity to engineer plants with desired traits. Major advances in plant transformation have occurred over the last few years. However, in many major crop plants, serious genotype limitations still exist. Transformation of some agronomically important crop plants continues to be both difficult and time consuming.
For example, it is difficult to obtain a culture response from some maize genotypes. Typically, a suitable culture response has been obtained by optimizing medium components and/or explant material and source. This has led to success in some genotypes. While, transformation of model genotypes is efficient, the process of introgressing transgenes into production inbreds is laborious, expensive and time consuming. It would save considerable time and money if genes could be more efficiently introduced into and evaluated directly into inbreds. Accordingly, methods are needed in the art to increase transformation efficiencies of plants.
Influencing cell cycle and cell division can also affect various developmental pathways in a plant. Pathways of interest include those that influence embryo development. The AP2/ERF family of proteins is a plant-specific class of putative transcription factors that have been shown to regulate a wide-variety of developmental processes and are characterized by the presence of a AP2/ERF DNA binding domain. The AP2/ERF proteins have been subdivided into two distinct subfamilies based on whether they contains one (ERF subfamily) or two (AP2 subfamily) DNA binding domains.
One member of the AP2 family that has been implicated in a variety of critical plant cellular functions is the Baby Boom protein (BBM). The BBM protein from Arabidopsis is preferentially expressed in seed and has been shown to play a central role in regulating embryo-specific pathways. Overexpression of BBM has been shown to induce spontaneous formation of somatic embryos and cotyledon-like structures on seedlings. See, Boutiler et al. (2002) The Plant Cell 14:1737-1749. Thus, members of the AP2 protein family promote cell proliferation and morphogenesis during embryogenesis. Such activity finds potential use in promoting apomixis in plants.
Apomixis refers to the production of a seed from the maternal ovule tissue in the absence of egg cell fertilization (Koltunow (1995) Plant Physiol 108:1345-1352). Apomixis is a valuable trait for crop improvement since apomictic seeds give rise to clonal offspring and can therefore be used to genetically fix hybrid lines. The production of hybrid lines is intensive and costly. Production of seed through apomixis avoids these problems in that once a hybrid has been produced, it can be maintained clonally, thereby eliminating the need to maintain and cross separate parent lines. The use of apomictic seeds also eliminates the use of cuttings or tissue culture techniques to propagate lines, reduces the spread of disease which are easily spread through vegetative-propagated tissues and in many species reduces the size of the propagule leading to lower shipping and planting costs. Methods are therefore needed for the efficient production of apomictic seed.
Members of the APETALA2 (AP2) family of proteins play critical roles in a variety of important biological events including development, plant regeneration, cell division, etc. Accordingly, it is valuable to the field of agronomic development to identify and characterize novel AP2 family members and develop novel methods to modulate embryogenesis, transformation efficiencies, oil content, starch content and yield in a plant.